Dynamically pin cloth when animating
Tried dynamically switching pinned vert groups - that resets simulation.
Tried dynamically changing vertex weights inside group - that resets simulation too.
And the bake wouldn't work at all, that was clear from beginning.

The workflow I suggest is to


simulate part of your cloth animation with one set of pinned verts and then save the animation as .mdd mesh cache.
Then apply the cloth modifier, add another and simulate with different set of pinned verts. Export to .mdd
After you have all the parts, load them to Mesh Cache modifiers and blend them with influence. With good blending you can mask some of the discontinuities that happened while switching simulations.


Since any motion between simulations will be canceled you would need to adjust with force fields and plan the transitions carefully. But that's probably the best you can do.